This invention relates to synthetic polymer fibers useful for reinforcing matrix materials, and more particularly to fibers having micro-mechanically-deformed morphologies useful for enhanced performance in matrix materials such as asphalt, rubber, plastic, or in such matrix materials such as ready-mix concrete, shotcrete, bituminous concrete, gypsum compositions, or other hydratable cementitious compositions; to matrix compositions containing such fibers; and to methods for treating fibers and for modifying matrix materials.
Although the fibers of the present invention are believed suitable for reinforcing a number of matrix materials, such as adhesives, asphalt, composites, plastic, rubber, etc. and structures made therefrom, they are primarily intended for reinforcing hydratable cementitious compositions such as ready-mix concrete, precast concrete, masonry concrete, shotcrete, bituminous concrete, gypsum compositions, gypsum- and/or Portland cement-based fireproofing compositions, and other hydratable cementitious compositions. A major purpose of the fibers of the present invention is reinforcing concrete (e.g., ready-mix, shotcrete, etc.) and structures made from these. The task of reinforcing matrix materials such as these poses one of the greatest challenge for designers of reinforcing fibers.
Concrete is made using a hydratable cement binder, a fine aggregate (e.g., sand), and a coarse aggregate (e.g., small stones, gravel), and is consequently a brittle material. If a concrete structure is subjected to stresses that exceed its maximum tensile strength, then cracks can be initiated and propagated in the concrete. The ability of a concrete structure to resist crack initiation and crack propagation can be understood with reference to the xe2x80x9cstrengthxe2x80x9d and xe2x80x9cfracture toughnessxe2x80x9d of the fibers.
Fiber xe2x80x9cstrengthxe2x80x9d relates to the ability of a cement or concrete structure to resist crack initiation. In other words, fiber strength is proportional to the maximum load sustainable by the structure without cracking, and is a measurement of the minimum load or stress (e.g., the xe2x80x9ccritical stress intensity factorxe2x80x9d) required to initiate cracking in that structure.
On the other hand, xe2x80x9cfracture toughnessxe2x80x9d relates to the specific xe2x80x9cfracture energyxe2x80x9d of a cement or concrete structure. This concept refers to the ability of the structure to resist propagationxe2x80x94or wideningxe2x80x94of an existing crack in the structure. This toughness property is proportional to the energy required to propagate or widen the crack (or cracks). This property can be determined by simultaneously measuring the load required to deform or xe2x80x9cdeflectxe2x80x9d a fiber-containing concrete (FRC) sample at an opened crack and also measuring the amount or extent of deflection. The fracture toughness is therefore determined by dividing the area under a load deflection curve (generated from plotting the load against deflection of the FRC specimen) by its cross-sectional area.
In the cement and concrete arts, fibers have been designed to increase the strength and fracture toughness in reinforcing fibers. Numerous fiber materials can be used for these purposes, such as steel, synthetic polymers (e.g., polyolefins), carbon, nylon, aramid, and glass. The use of steel fibers for reinforcing concrete structures remains popular due to the inherent strength of the material. However, one of the concerns in steel fiber product design is to increase their xe2x80x9cpull outxe2x80x9d resistance because this increases the ability of the fiber to defeat crack propagation. In this connection, U.S. Pat. No. 3,953,953 of Marsden disclosed fibers having xe2x80x9cJxe2x80x9d-shaped ends for resisting pull-out from concrete. However, stiff fibers having physical deformities may cause entanglement problems that render the fibers difficult to handle and to disperse uniformly within a wet concrete mix. More recent designs, involving the use of xe2x80x9ccrimpedxe2x80x9d or xe2x80x9cwave-likexe2x80x9d polymer fibers, may have similar complications, depending on the stiffness of the fiber material employed.
U.S. Pat. No. 4,414,030 of Restrepo disclosed the use of microfibrillated polyolefin filaments that are oriented in all spatial directions by subjecting fibrillated ribbons to air, thereby spreading out the separate fibers, and then feeding these separated fibers into a mortar mixing machine fitted with a high-speed propeller to blend the mortar components and fibrous materials together. The mechanical shredding action which takes place in the mixing operation causes the ribbons to become further fibrillated, such that the ribbon fibrils are broken apart into individual filaments having a branched structure with microfibrils outwardly projecting along their length. The projected microfibrils are somewhat curled in shape and perform as anchoring elements or xe2x80x9chooksxe2x80x9d within the cement hardened matrix. It is generally believed that side branches or xe2x80x9chooksxe2x80x9d can act to resist fiber dislodgment or pull-out from the cement matrix and present enlarged surface area for anchoring within concrete. The physical branched fiber structure would appear to create entanglement problems that would render handling and dispersion within a wet concrete mix somewhat difficult to achieve.
U.S. Pat. No. 5,753,368 of Berke et al. taught fibers having a glycol ether-based coating for enhancing bond strength of the fibers within concrete. Berke et al. further taught that the fibers could be bundled using mechanical or chemical means, and that the fibers could be introduced into a cement composition using packaging technology to facilitate mixing and dispersion within concrete. This technology may be applied to varieties of fibers and shapes to enhance pull out resistance while facilitating uniform dispersion within the concrete mix.
U.S. Pat. No. 5,298,071 of Vondran discussed the problem of achieving a uniform dispersal of fibers within a wet cement mix. Vondran noted that fibers were typically added to the mixer with the cement, sand, aggregate, other admixtures, and water. His approach was to add fiber precursors (e.g., steel fibers and polyolefin in the form of extruded monofilament or fibrillated sheet fiber) and cement clinker to a ball mill grinder and to obtain a hydratable mixture comprising interground fibers in a dry hydratable cement powder that could then be used for making the concrete structure.
It is readily observed that Vondran""s clinker/fiber-intergrinding method (hereinafter the xe2x80x9cVondran methodxe2x80x9d) purports to achieve quick fiber wetting and uniform dispersion without the balling and clumping found when adding the fiber components separately into concrete. The present inventors, however, observe that the Vondran method teaches that xe2x80x9cfiber precursorsxe2x80x9d are combined with cement clinker particles into a ball mill cement grinder, and that this process provides fibers that are xe2x80x9cattenuated, roughened and abraded by the action of the clinker particles and the grinding elements on the fiberxe2x80x9d (See U.S. Pat. No. 5,298,071 at column 2, lines 58-66). This process purportedly results in improved mechanical bonding between the cement and fibers.
In the present invention, however, the inventors seek to improve the pull-out resistance of fibers from concrete while avoiding the kinds of mechanical or physical fiber attributes that might otherwise impede the ability of the fiber to be introduced into, and uniformly dispersed within, the concrete mix. The present inventors believe that the clinker intergrinding process of Vondran results in cement particles being ground into, and embedded in, the fiber surface. Moreover, the deep-abrading action of the cement clinker may be undesirable because the fibers will tend to clump during humid conditions (e.g., storage, shipment) due to the hydrating cement particles. Furthermore, fibers can not be interground with clinker at high volumes using ball mill machinery in an clinker-intergrinding process because the fibers would potentially clog the classifier unit used in such mills for separating ground cement particles from the grinding operation. The present inventors have also discovered that fibers interground in ball mill operations using clinker are severely abraded, and, in effect, are shredded to the point at which their mechanical integrity, for purposes of reinforcing concrete, is defeated. Such clinker-interground fibers, whether by abrasion and/or impact of clinker material, lose mechanical resistance to pull-out from concrete (i.e., fracture toughness) because the fiber bodies and ends are shredded or devastated by the clinker/fiber intergrinding operation.
The terms xe2x80x9cshreddedxe2x80x9d or xe2x80x9cshreddingxe2x80x9d are used herein to refer to the tearing-apart of the fiber body into smaller elongated pieces. The concept of xe2x80x9cshreddingxe2x80x9d as used herein is not equated herein with the concept of xe2x80x9cfibrillationxe2x80x9d. The concept of fibrillation may be seen to occur where a multifilament fiber, comprised of two or more strands or fibrils are adhered or bonded together, is separated into its component strands or fibrils. On the other hand, xe2x80x9cshreddingxe2x80x9d is defined for present purposes as the act of breaking a fiber down (whether monofilament or multifilament) into pieces smaller than the constituent strands or fibrils.
In view of the disadvantages of the prior art as discussed above, the present inventors believe that a novel fiber for reinforcing matrix materials, and in particular hydratable cementitious materials such as concrete and shotcrete, are needed. Also needed are novel methods for making such fibers and for modifying such matrix materials.
In contrast to the above-described prior art fibers and methods for manufacturing reinforcing fibers, the present invention provides fibers which are micro-mechanically-deformed such that the fibers are flattened and have surface deformations for improved contact with the matrix material. Fibers of the invention are mechanically-flattened to provide macro-level deformations in terms of varying width and/or thickness dimensions within fiber lengths, but are also xe2x80x9cdiastrophicallyxe2x80x9d deformed to provide micro-level deformations (e.g., microscopic material displacements) on the fiber surface. This is achieved while avoiding the obliterative clinker intergrinding process of the prior art.
The term xe2x80x9cdiastrophic,xe2x80x9d as used herein is defined in Webster""s Third New International Dictionary (Merriam-Webster Unabridged Dictionary, Springfield, Mass.) as follows: an adjective xe2x80x9cof, having reference to, or caused by diastrophism.xe2x80x9d The term xe2x80x9cdiastrophism,xe2x80x9d in turn, is defined in this Webster""s dictionary as xe2x80x9cthe process of deformation that produces in earth""s crust its continents and ocean basins, plateaus and mountains, folds of strata, and faultsxe2x80x94.xe2x80x9d
The present application, therefore, borrows geological terminology in describing xe2x80x9cmicro-diastrophicxe2x80x9d synthetic fibers which have a microscopic surface xe2x80x9cdiastrophismxe2x80x9d. After application of the flattening processes of the invention, a number of physical deformations or material displacements caused or induced in the fibers can be seen under the microscope to resemble geological morphologies or phenomena. For example, the microscopically viewed surfaces of the treated fibers have irregularly and randomly elevated portions or ridges resembling islands, continents, plateaus, and mountains; and there can also be detected equally random folds of strata, faults (or fissures), and other physical displacements of fiber material. These microscopic deformation irregularities appear randomly on the surface of a given fiber, as well as from fiber to fiber.
Thus, the term xe2x80x9cmicro-diastrophicxe2x80x9d is appropriate for describing the micro-level deformations or physical displacements of exemplary fibers of the present invention. The term xe2x80x9cmicro-diastrophismxe2x80x9d also appears to describe the three-dimensional morphological changes achieved by the novel methods of the invention. These morphological changes may be achieved by subjecting synthetic polymer material (preferably a polypropylene, polyethylene, or mixture thereof) to a compressive force. An exemplary compressive force may be achieved by using at least one roller, and preferably opposed rollers to compress the fibers to induce irregular and random microscopic surface deformations that are described herein as diastrophic; this process is very different from superficially embossing or crimping fibers. Alternatively, though less preferably, the effect may be achieved by using a ball mill (without the use of cement clinker as taught by Vondran et al). The stress forces on the fibers should be sufficient to flatten the fibers in a manner to increase and vary (within the length of the fiber) the fiber width dimension, thickness dimension, or both; and to cause or induce micro-diastrophism in the fiber surface as mentioned above. The micro-diastrophism in the fiber surface causes an increase in the total fiber surface area that can be placed into contact with the matrix material. The micro-diastrophic surface deformities should be achieved without substantially shredding the elongated body or end portions of the fibers (e.g., without cement particles being embedded in, with attendant abrasion of, the fiber surface), although a small amount of fibrillation or shredding at the extreme fiber ends may be tolerated within the spirit of the present invention.
One advantage of the fibers of the invention is their ability to provide strong bonds with the matrix material (e.g., concrete). This is believed to arise from the fibers having a variable width and/or thickness dimension(s), and enhanced bonding surface due to micro-diastrophism in the fiber surface. These advantages are provided while avoiding a substantial increase in fiber-to-fiber entanglement or clumping which would otherwise be expected to arise during or after mixing into the matrix material. Another advantage of the invention is that, in the absence of using the prior art clinker-intergrinding method, the fibers and methods of the present invention are substantially free of embedded cement/clinker particles and the abrasive and obliterative shredding caused by the prior art clinker-intergrinding operation.
Thus, the present invention provides high performance fibers and methods for reinforcing matrix materials against cracks without entailing the problems of prior art reinforcing fibers. Exemplary fibers of the invention comprise a plurality of mechanically-flattened fibers having generally elongate bodies, opposed body ends defining a fiber length, said fiber bodies have varied width and/or thickness dimensions and having micro-diastrophic surface deformities. Matrix materials and structures comprising such fibers are also disclosed and claimed. An exemplary method of the present invention for manufacturing fibers comprises providing a plurality of synthetic polymer fibers, and mechanically flattening these fibers to the extent that the fibers, after said mechanical flattening, have a varied width and/or thickness dimension and micro-diastrophism. Further advantages and features of the invention are further described in detail hereinafter.